1. Field of the Invention
This invention relates generally to the field of disc drive data storage devices, and more particularly, but not by way of limitation, to a method and apparatus for providing feed forward compensation to improve head positioning in a disc drive.
2. Discussion
Modern hard disc drives comprise one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Information is stored on the discs in a plurality of concentric circular tracks by an array of transducers, or heads (usually one per disc surface) mounted for movement to an electronically controlled actuator mechanism. The storing of information on the discs is referred to as xe2x80x9cwritingxe2x80x9d and the subsequent retrieval of information from the discs is called xe2x80x9creadingxe2x80x9d.
Presently, the most commonly used type of actuator mechanism is the rotary voice coil actuator, also referred to as a rotary moving-coil actuator. With this type of actuator, the transducers used to write and read data are mounted via flexures at the ends a plurality of head arms which project radially outward from a substantially cylindrical actuator body. The actuator body is journaled via ball bearing assemblies to rotate about a pivot shaft which is mounted to the disc drive housing at a position closely adjacent the outer extreme of the discs. The pivot shaft is intended to be in parallel with the axis of rotation of the spindle motor and the discs. The transducers will thus move in a plane parallel with the surfaces of the discs.
A coil is mounted on the side of the actuator body opposite the head arms so as to be immersed in the magnetic field of an array of permanent magnets which are in turn mounted to the disc drive housing. When controlled DC current is passed through the coil, an electromagnetic field is set up which interacts with the magnetic field of the permanent magnets and causes the coil to move relative to the permanent magnets in accordance with the well-known Lorentz relationship. As the coil moves relative to the permanent magnets, the actuator body pivots about the pivot shaft and the heads are moved across the disc surfaces.
Control of the movement of the heads is achieved with a closed loop servo system such as disclosed in U.S. Pat. No. 5,262,907 entitled HARD DISC DRIVE WITH IMPROVED SERVO SYSTEM, issued to Duffy, McKenzie, Heydari and Woods, assigned to the assignee of the present invention and incorporated herein by reference. In such a system, position or servo information is prerecorded on at least one surface of one of the discs. The servo system can be either a xe2x80x9cdedicatedxe2x80x9d servo system, in which one entire disc surface is prerecorded with the servo information and a dedicated servo head is used to constantly read the servo information, or an xe2x80x9cembeddedxe2x80x9d servo system, in which servo information is interleaved with user data and intermittently read by the same heads used to read and write the user data.
With either a dedicated or embedded servo system, it is common that the servo circuitry produce a servo/position error (SPE) signal which is indicative of the position of the head relative to the center of a track. The identity of the particular track, as well as other information relating to the circumferential position of the head on the track, is included, along with other information, in the prerecorded servo information. Thus, when the heads are following a desired track, the SPE will ideally have a zero value. The SPE is fed back to circuitry used to control current through the coil of the actuator and any tendency of the heads to deviate from true track center causes the SPE to change from its zero value accordingly.
It is known that the SPE can contain certain low frequency components as a result of defects in the disc drive; more particularly, if the disc is not centered precisely over the axis of rotation, the circular tracks may have an eccentric shape with respect to the axis of rotation. This will result in a low frequency component existing within the SPE at a frequency equal to the revolutions per second of the disc drive (for example, a 90 Hz signal will appear in an SPE plot for such a disc drive operating at 5400 revolutions per minute, or 90 revolutions per second). This low frequency component will comprise a repeatable runout (RRO) signal for which the servo system will continually make corrections, resulting in an oscillation of the heads at the RRO frequency as the discs move in relation to the actuators.
The effects of such RRO signals on disc drive performance are well known and the prior art discloses several approaches that have been taken to minimize these effects, including U.S. Pat. No. 4,594,622 issued to Wallis, which discloses integrating the actuator control signal from the transducer twice in order to generate a pseudo head position signal which is summed with the SPE. The resultant signal is then twice differentiated and the result is stored and subsequently combined with the actuator control signal. In this way, a corrected SPE signal is provided to the actuator in order to correct for the RRO signal.
Additionally, U.S. Pat. No. 4,616,276 issued to Workman discloses the calculation of a group of tap weights for a set of sine and cosine functions to generate a modified actuator control signal. The tap weight calculations converge to a final value in accordance with a convergence function as SPE sample values are provided from the actuator and each tap weight is multiplied by its appropriate sine or cosine function. The resultant products are summed to provide a correction signal which is combined with the SPE to provide the modified control signal to the actuator.
While the prior art has been operative, as spindle speeds continue to increase and track spacings on the discs continue to decrease, there remains a continual need for improved methods for minimizing the effects of the low frequency RRO component in an SPE signal in an efficient and effective manner.
The present invention provides an improved method and apparatus for minimizing the effects of repeatable runout (RRO) signals in an SPE in a disc drive. More particularly, the present invention provides a method and apparatus of a disc drive which obtains a measurement estimate through the sampling and accumulating of data from the SPE during track following over a mechanical revolution of the drive, the measurement estimate providing a characterization of the repeatable runout signal in the servo position error signal. A correction estimate is updated by the measurement estimate, and the correction estimate is added to the servo position error signal to provide an adjusted servo position error signal. The disc drive uses the adjusted servo position error signal to generate a correction signal for adjusting the position of the actuator. The measurement estimate is reset after each update of the correction estimate and a new measurement estimate is generated during each subsequent rotation of the disc drive during track following mode. A discrete fourier transform (DFT) is used in a preferred embodiment of the invention to obtain the measurement estimate.
An object of the present invention is to provide a method and apparatus for minimizing the effects of repeatable runout (RRO) signals in an SPE in a disc drive.
Another object of the present invention, while achieving the above stated object, is to provide a method and apparatus that characterizes the RRO signal during each mechanical rotation of the disc.
Still another object of the present invention, while achieving the above stated objects, is to generate a compensation signal that is updated by the characterized RRO signal.
Another object of the present invention, while achieving the above stated objects, is to use the compensation signal to generate an adjusted SPE signal which is used to correct the position of the actuator.
Yet another object of the present invention, while achieving the above stated objects, is to compensate for vibrations due to disc imbalance or off-center mounting and for such vibrations provided from adjacent disc drives.
Other objects, advantages and features of the present invention will be apparent from the following description when read in conjunction with the drawings and appended claims.